Twin studies have been one of the most important sources of information about the role of familial-genetic factors in the etiology of schizophrenia. However, no population-based twin study of schizophrenia has been initiated in over 20 years, during which time major advances have been made in both the methodology of psychiatric field studies and in the genetic-statistical analysis of twin data. This is a proposal to support a pilot field study of twins with broadly defined schizophrenia, based on the recently created Swedish Psychiatric Twin Registry (SPTR). The SPTR, an epidemiologic, population-based registry formed by merging the National Twin and Psychiatric Registries in Sweden, contains 969 twin-probands with a diagnosis of non-affective psychosis (NAP), of whom 403 have been diagnosed as schizophrenic by narrow Swedish criteria. The population registry in Sweden will be used to locate current addresses for all twins. In order to gain the necessary experience to plan and execute the study of this entire unique cohort, the current application seeks support for field interviews for 50 randomly selected proband twins and their co-twins. No previous population-based twin study of psychosis has utilized a blindly administered structured psychiatric interview. Because of its large size and excellent epidemiologic characteristics, when completed, this study will be in a unique position to clarify a number of critical issues about the familial-genetic transmission of schizophrenia.